In an unprecedented gesture of solidarity with Syria's protest movement, Britain's ambassador to Damascus has joined other western envoys at a vigil for a prominent human rights activist believed to have been killed under torture by security forces.

The co-ordinated attendance by the eight top diplomats, including the US ambassador Robert Ford and representatives from France, Germany, Canada, Japan, the Netherlands and the EU, is the most public show of international support for the protesters yet.

Simon Collis, the British ambassador, said his attendance at Giyath Matar's vigil on Sunday was to "express condolences" after the family welcomed the diplomats' request to attend.

"It is important to show Giyath's family and Syrians that the world has noticed what is going on and to increase awareness of the wider situation in Syria," said Collis. "I spoke to his father: the family are very keen that what happened is known."

Attending the vigil was a novel approach by foreign officials as Syria continues to ignore diplomatic pressure. It was intended to send a message to the regime of President Bashar al-Assad, which, despite mounting international pressure, has continued its violent crackdown against protesters.

More than 2,600 Syrians have been killed, according to the United Nations, and thousands detained and tortured.

"The broader message to the regime is that this killing and torture must end," Collis said. "Giyath was so clearly associated with peaceful protests and for somebody like that to die in custody is outrageous."

Matar, 26, an activist in Daraya, near Damascus, was picked up by plainclothes security agents on 6 September. Four days later, his corpse was returned to his family bearing bruising and other marks. They were told he had been "shot by armed gangs", Amnesty International reported, but sources said his family were told he would be returned as "shwarma" – a reference to cooked meat.

He had been at the forefront of non-violent protests in Daraya, organising flowers and water to hand out to soldiers. "He was one of the most active in the town and at the forefront of the non-violent resistance there," said Radwan Ziadeh, a US-based human rights activist from Daraya.

Local activists said two of Matar's uncles were temporarily detained on Wednesday, as security forces continued a countrywide sweep. They killed at least one person in Homs, stormed 10 villages in north-western Jebel Zawiyah, and carried out door-to-door arrests in the eastern region, activists said. Security forces were also reported to have destroyed the house of Colonel Riyad al-Assad, who defected in July and proclaimed himself the head of the Syrian Free Army.

Ford has been praised by protesters for his trips to restive towns, including a visit to Hama at the start of July, when security forces had withdrawn from the city. He has also written critical posts on Facebook.

His actions have attracted the wrath of the regime. Damascus ordered limitations on all diplomatic travel outside Damascus, and preventing contact with protesters.

Hours after the envoys left the vigil, troops fired tear gas at mourners.

Collis, who said the diplomats were given flowers by people at the vigil, said the visit was not unusual. "British diplomacy has always been about talking to people and using publicity to increase understanding," he said. "Any abnormality is by the Syrian regime, which is trying to limit the travel of diplomats and stop contact with anyone but its own members which makes visits like this important."

At least 95 people have died in custody, according to Amnesty. Human rights groups have expressed particular concern for Yahya Shurbaji, one of four activists detained at the same time as Matar who remain in custody. Five others from Daraya have not been heard from since their arrests in July and August. "It is clear that these human rights activists from Daraya are in grave danger," said Philip Luther, Amnesty's deputy director for the Middle East and North Africa.